1. Field of the Invention
This invention relates to a method for processing radiation images wherein, when radiation images are recorded and reproduced, read-out image signals, which have been obtained by reading out the recorded radiation images, are processed such that an image having good image quality may be obtained ultimately. This invention particularly relates to a method for processing radiation images, wherein each of radiation images is stored on a stimulable phosphor sheet, which is capable of storing energy from radiation thereon during its exposure to the radiation, the stimulable phosphor sheet on which the radiation image has been stored is then exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon, the emitted light is photoelectrically detected and converted into an image signal representing the radiation image, and the image signal is thereafter processed and used in reproducing a visible radiation image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264 and 4,346,295 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
Also, with the radiation image recording and reproducing systems which use stimulable phosphor sheets, after the radiation image, which has been stored on a stimulable phosphor sheet, is converted into an electric signal, appropriate image processing can be carried out on the electric signal. The electric signal, which has been obtained from the image processing, may then be used in reproducing a visible image of the radiation image on a recording material, such as photographic film, or on a display device, such as a CRT display device. In this manner, a visible radiation image can be obtained which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. This is very advantageous in practice.
However, with the aforesaid radiation image recording and reproducing systems, when an object is exposed to radiation during the recording of a radiation image of the object on a stimulable phosphor sheet, scattering of the radiation (Thomson scattering or Compton scattering) is caused to occur by elastic collision or electromagnetic interaction between the radiation and substances constituting the object. The radiation is also scattered by elastic collision or electromagnetic interaction between the radiation and substances constituting the radiation image recording apparatus or between the radiation and substances constituting a cassette in which the stimulable phosphor sheet is housed. The radiation, which has thus been scattered, travels three-dimensionally in random directions and impinges upon the stimulable phosphor sheet. If the stimulable phosphor sheet is exposed to the principal radiation, which has passed through the object and which carries the image information of the object, and the scattered radiation, the contrast and the sharpness of the radiation image stored on the stimulable phosphor sheet with the principal radiation, which has passed through the object and which carries the image information of the object, will become bad.
Various attempts have heretofore been made in order to eliminate the adverse effects of the scattered radiation. As one such attempt, a method has heretofore been known wherein a grid member for absorbing the radiation is located between an object and a stimulable phosphor sheet. The grid member is constituted of a plurality of lead plates, which have a thickness of, for example, 1 mm or less and which are combined in a grid pattern or in a row pattern. In cases where the grid member is located in the manner described above, the scattered radiation, which travels in random directions, is absorbed by the lead plates of the grid member.
However, in cases where ordinary radiation images of human bodies are recorded, the amount of the scattered radiation is approximately as large as the amount of the principal radiation, which has passed through the object and which carries the image information of the object. Therefore, even if the grid member is located between the object and the stimulable phosphor sheet, the scattered radiation cannot be completely eliminated by the grid member. Also, if the grid member is used to eliminate the scattered radiation, part of the principal radiation, which has passed through the object and which carries the image information of the object, will be blocked by the grid member. Therefore, the amount of the principal radiation, which impinges upon the stimulable phosphor sheet, becomes small. As a result, the signal-to-noise ratio (S/N ratio) of the read-out image signal detected from the stimulable phosphor sheet becomes low, and the graininess of a visible image reproduced from the read-out image signal cannot be kept good.